Magnesium base alloy



Patented Dec. 25, 1934 UNITED STATES PATENT OFFICE MAGNESIUM BASE ALLOYNo Drawing.

Application May 18, 1934,

Serial No. 726,289

sciaims.

This invention relates to new and improved magnesium base alloys and itis particularly concerned with that class of alloys which contain noaluminum. r

' Magnesium base alloys have been extensively employed in structureswhere it is essential to combine light weight with a moderate strength.

.The alloys have been used in both'cast and wrought form, the particularform used for a given application being determined by the type ofservice expected and the design of the article.v

Specific alloys-have, furthermore, been developed that areparticularlyadapted to casting or working since each of these methods offabrication has peculiarities for which allowance must be 1 made if thebest results are to be obtained. Casting alloys have receivedconsiderable attention because of the difficulties encountered inworking magnesium and its alloys. An alloy used for 2 castings thatpossesses a high strength compared with other cast magnesium basealloys'is one containing aluminum as the sole or chief alloyingingredient. The aluminum content is generally in excess of '1 per cent.Such an alloy casts well 5 and is furthermore susceptible to improvementin strength through'solution heat treatment and in In spite of theseadvantageous properties,

however, the alloy can not be used in the presence of salt water withoutbeing severely at- 9 tacked with consequent deterioration and ultimatefailure. This limitation seriously affects the field of utility of thealloy since there are many potential applications for magnesium basealloys in naval craft or other structures subjected to salt water andspray. Another drawback of magnesium-aluminum alloys lies in the greatersusceptibility to corrosive attack in alloys that have been heattreated. The eifort to attain a maxi- "mum strength is attended by aneven lower corle rosion resistance than possessed by the alloy in the ascast condition.

It is an object of my invention to produce a magnesium base castingalloy that has a corrosion resistance superior to that of the commercialmagnesium-aluminum alloy and yet possesses a moderately high strength.Another object is to make an alloy that can be strengthened by heattreatment without deleterious eifect upon the corrosion of tin,'4 percent of zinc, 0.3 per cent of man- 1 to 15 per cent of tin, 0.1 to 10per cent of zinc,

0.1 to 2 per cent of manganese and 0.1 to 1.5 per cent of silicon. Analloy of this composition possesses suiiicient strength for allo'rdinarypurposes where magnesium base alloys may be used. I have 5 found thatsuch an alloy is much superior to magnesium-aluminum alloys in resistingattack by salt water, particularly in the heat treated and agedcondition. The heat treatment commonly employed to improve the strengthof magnesium- 10 aluminum alloys is effective in improving my alloyalso. The customary aging treatment may likewise be used to effect astill further increase in strength.

While the alloy within the range of composition above stated possessesthe described properties of strength and corrosion resistance, I havefound it advantageous under most conditions to use from about 4 to 10per cent of tin, 2 to 7 per cent of zinc, 0.2 to 1 per cent of manganeseand 0.1 to 0.75 per cent of silicon. An alloy composition thatillustrates my invention and represents preferred proportions of theseveral elements is one that consists of about 5 per cent ganese .and0.2 per cent of silicon; the balance being magnesium of commercialpurity. This alloy in the form of a sand casting had a tensile strengthof about 20,900 pounds per square inch, 2. yield strength of about 9,500pounds per square inch and an elongation of about 4 per cent in 2inches. After heat treating this sand cast alloy for 16 hours at 482 C.and quenching, the tensile strength-was increased to about 24,000 poundsper square inch and the elongation to 5.2 per cent, the yield strengthremaining practically unchanged. Upon aging the heat treated. alloy for8 hours at C., the tensile strength was further increased to about26,400 pounds per square inch, the yield strength to 16,900 pounds persquare inch and the elongation diminished to 3.5 per cent. Incomparison, typical mechanical properties of the magnesium-aluminumalloy containing 8 per cent aluminum, the amount commonly used incasting alloys, are as follows. In the as cast condition it has atensile strength of about 22,000 pounds per square inch, and anelongation of about 2 per cent. 'When heat treated the alloy has astrength in the neighborhood of 32,000 pounds per square inch and anelongation of 6 per cent. If aged after heat treatment, the tensilestrength remains practically unaltered but the yield strength isconsiderably increased, and the elongation drops toaboutiipercent. 55

While the magnesium-aluminum alloy has as high or a higher strength thanalloys of my invention under normal conditions, the superiority of thealloys herein disclosed is conspicuous under corrosive attack by saltwater. In a severe corrosion test comprising subjecting specimens of thealloy to a spray of 20% NaCl solution for more than a month, an alloycomposed of aboutv 5 percent tin, 4 per cent zinc, 0.3.per centmanganese and 0.2 per cent silicon, the balance being magnesium, in theas cast condition lost only about 28 per cent in strength. In the heattreated condition it lost only about 20 per cent in strength while theheat treated and aged alloy lost only 12 per cent in strength. Undersimilar conditions a magnesium-aluminum allow of the kind describedherein above loses from 20 to 25 per cent in strength. The absence ofany deleterious effect of heat treating and aging on the corrosionresistance of the magnesium base alloys containing tin, zinc, manganeseand silicon is contrary to the usual experience with aged alloys, and myalloys for this reason afford a peculiar advantage in 'ofiering themaximum attainable strength combined with a maximum corrosionresistance. Such a fortuitous combination of properties is ofconsiderable importance, particularly on aircraft operating on or nearsalt water.

To secure the maximum strength in my alloys, they should be heated to atemperature above about 400 C. and below the point of incipient fusionfor a sufllciently long period of time in order to get the practicalmaximum amount of room temperature. increase the strength of the alloy,it may be heated at a temperature of about C. for

several hours.

The alloy may be compounded in the manner generally employed in makingmagnesium base I alloys, preferably by adding the alloying constituentseither as metallic elements or as rich alloys with magnesium.

The term magnesium as used herein and in I the appended claims refers tothe commercially obtainable metal containing the usual impurities Iclaim:

1. A magnesium base alloy composed of magnesium and about 1 to 15 percent of tin, 0.1 to 10 per cent of zinc, 0.1 to 2 per cent of manganeseand 0.1 to 1.5 per cent of silicon.

2. A magnesium base alloy composed of magnesium and about 4 to 10per-cent of tin, 2 to 7 per cent of zinc, 0.2 to l per cent of manganeseand 0.1 to 0.75 per cent of silicon.

3. A magnesium base alloy consisting of magnesium and about 5 percent oftin, 4 per cent of zinc, 0.3 per cent of manganese and 0.2 per cent ofsilicon.

ROY E. PAINE.

